Temperature Compensated Current Sensor using Reference Magnetic Field

ABSTRACT

A method is described to provide temperature compensation and self-calibration of a current sensor based on a plurality of magnetic field sensors positioned around a current carrying conductor. A reference magnetic field generated within the current sensor housing is detected by a separate but identical magnetic field sensor and is used to correct variations in the output signal due to temperature variations and aging.

CROSS REFERENCE TO PRIOR APPLICATION

This application is a divisional of U.S. application Ser. No. 10/905,509filed Jan. 7, 2005 and entitled “Current Sensor,” which claims thepriority of U.S. Provisional Application Ser. No. 60/481,906 filed Jan.16, 2004 and entitled “CURRENT SENSOR”. The subject matter of U.S.application Ser. No. 10/905,509 and U.S. Provisional Application Ser.No. 60/481,906 are incorporated herein by reference.

FEDERAL GOVERNMENT STATEMENT

This invention was made with Government support under contractDE-FG03-01ER83228 awarded by the Department of Energy. The Governmenthas certain rights to this invention.

FIELD OF THE INVENTION

The present invention relates to a clamp-on current sensor for measuringalternating and direct electrical current such as the current of ahigh-voltage power transmission line or a substation bus conductor.

DESCRIPTION OF THE PRIOR ART

A variety of current measurement techniques are known in the art,including current transformers, Rogowski coil transformers, resistiveshunts in series or in parallel with a current-carrying conductor,magnetic field point sensors, magnetic field line integral sensors, andline integral optical current sensors.

Current transformers consist of two or more windings, each windingconsisting of one or more turns of wire, around a continuous core havinga high magnetic permeability to concentrate the magnetic flux linesgenerated by current flowing in the windings. The ratio of turns in thewindings determines the turns ratio of the transformer. Clamp-on currenttransformers introduce a break in the core to permit its installationaround a current carrying conductor without breaking the conductor.Current transformers only respond to AC currents unless special stepsare taken to actively switch the magnetization direction or strength inthe core. Furthermore, at high voltages found in power transmissionsystems, current transformers become extremely large and heavy andcontain insulating mineral oil in order to satisfy the dielectricrequirements of the application.

A Rogowski coil consists of a coil winding placed around a core having amagnetic permeability similar to air. A current-carrying conductor ispassed through the coil, and generates an output voltage that isproportional to the time derivative of the current in the conductor. Thereal-time current can be estimated by time-integrating the signal fromthe Rogowski coil. Rogowski coils require an AC current to generate anoutput signal, and their output amplitude is proportional to thefrequency of the current.

A resistive shunt consists of a resistor connected to a current-carryingconductor in such a manner as to allow at least some of the conductorcurrent to pass through the resistive element. The resulting voltagedrop across the resistive element is a measure of the current flowingthrough the element. The resistive element can be placed in series withthe current-carrying conductor, whereby all the conductor current passesthrough the resistive element, or it can be placed in parallel with aportion of the current-carrying conductor, whereby it shunts a knownportion of the current away from the conductor. Resistive currentsensors can measure AC or DC currents, and are relatively easy to usewhen the currents to be measured are small, i.e. less than 100 Amperes.

Field sensors take advantage of the magnetic field generated by thecurrent in a conductor. By placing a point magnetic field sensor nearthe conductor, the sensor output signal is proportional to the currentin the conductor. By using a magnetic field sensor of the proper type,this current sensor can respond to AC or DC currents, and can have awide frequency response. Calibration is difficult to achieve or maintainwith this approach. Stray magnetic fields generated by other currentslocated nearby will also cause measurement errors.

Optical current sensors use the Faraday effect in an optical solid tochange the travel time, polarization state or optical phase of anoptical signal, in direct proportion to the magnetic field present alongthe optical path. By creating a closed optical path that encircles acurrent carrying conductor, the resulting signal is proportional to thecurrent, and is substantially immune to interfering magnetic fields fromother conductors, the position of the conductor relative to the sensorstructure, and the size of the conductor. The sensor can be made torespond to DC or AC currents, and it can have a high bandwidth. Opticalcurrent sensors are difficult to design as a clamp-on device, and theysuffer from high costs.

The most accurate current sensors take advantage of Ampere's law, whichstates that the line integral of the magnetic field along a closed pathencircling a current is proportional to the current. More importantly,the integral is not sensitive to the details of the path shape, thespatial distribution of the current within the closed integration path,or the presence of any currents that do not pass through the closedintegration path. The current transformer achieves this by having aclosed path of high permeability core. The Rogowski coil achieves thisby having a coil encircling the conductor with uniform turns per inchalong the winding. The optical current sensor achieves this by having anoptical sensor element encircling the conductor, such as a block ofglass through which a hole has been machined to allow a conductor topass through and in which the optical beam propagates in a closed pathencircling the hole, or an optical fiber that carries the optical signaland can be formed into a closed loop or loops around the currentcarrying conductor.

Current sensors that rely on one or two point magnetic field sensors donot approach a good approximation of Ampere's law, and are thereforeprone to inaccuracies due to the presence of external magnetic fieldsand the position of the conductor relative to the sensor(s).

Baker discloses in U.S. Pat. No. 5,493,211, issued Feb. 20, 1996, acurrent probe using a Hall sensor that can be calibrated by using aswitched coil to introduce a known current into the conductor under testand measuring the response of the Hall sensor. The response can be usedto calibrate the Hall sensor output in response to currents in theconductor under test. This method requires the induction of a testcurrent into the conductor being monitored, which may be difficult whenhigh currents are being measured on a power line. Berkcan discloses inU.S. Pat. No. 5,459,395 issued Oct. 17, 1995, and U.S. Pat. No.5,438,257 issued Aug. 1, 1995, a method of using a coil to generate amagnetic field that is sensed by two point magnetic field sensors, ortwo line integrating magnetic field sensors, to create a calibration ofthe ratio of the responses from the pair of sensors. The ratio is thenused to calibrate the response of the sensor pair to the magnetic fieldgenerated by the current flowing through the conductor under test. Thismethod suffers from being sensitive to stray magnetic fields in thevicinity of the conductor under test, resulting in erroneous readingsfrom the sensor pair.

Several attempts have been made to provide separate point sensors tocompensate for the presence of external magnetic fields not generated bythe current in the conductor of interest. Marx disclosed in U.S. Pat.No. 5,124,642 issued Jun. 23, 1992 the use of two coil sensors placed onopposing sides of a current carrying conductor to measure the current.The two coils are oppositely polarized, and the two signals aredifferenced to provide a signal that is substantially proportional tothe time derivative of the current in the conductor, and less sensitiveto stray magnetic fields. Friedl discloses in U.S. Pat. No. 4,894,610issued Jan. 16, 1990, the use of two or more coil sensors to measure thecurrent in a conductor while reducing the errors caused by straymagnetic fields. Arnoux et al. disclose in U.S. Pat. No. 6,215,296issued Apr. 10, 2001 the use of two point magnetic field sensors tomeasure the current in a conductor, with one sensor being shielded orotherwise located to provide compensation for external stray magneticfields. Lienhard discloses in U.S. Pat. No. 4,559,495, issued Dec. 17,1985, the use of two sensors located near a conductor to measure thecurrent carried by the conductor. All of the disclosed techniques areattempts to approximate Ampere's law with two sensors. This approachdoes not eliminate errors due to stray magnetic fields, and requirescareful geometric stability of the sensor locations to maintaincalibration.

Berkcan discloses in U.S. Pat. No. 5,438,257 issued Aug. 1, 1995 andU.S. Pat. No. 5,463,313 issued Oct. 31, 1995, the use of two pointsensors or two line integral sensors to measure the current in aconductor. The sensors are mounted near the conductor, and the ratioresponse of the two sensors to the conductor current is calibratedduring construction. A separate coil is also disclosed that is driven byan adjustable current to reduce or null the magnetic field at thesensors. Other than reducing the magnetic flux at the sensors, there isno clear advantage of this approach.

Hall or Magneto-resistive sensors have been coupled with a core having ahigh magnetic permeability to focus the magnetic flux lines through thesensor. Marasch et al. in U.S. Pat. No. 6,759,840 issued Jul. 6, 2004,Becker et al. in U.S. Pat. No. 6,1 75,229 issued Jan. 16, 2001, McLymanin U.S. Pat. No. 5,103,163 issued Apr. 7, 1992, Radosevich et al. inU.S. Pat. No. 6,545,456 issued Apr. 8, 2003, Baran et al. in U.S. Pat.No. 4,857,837 issued Aug. 15, 1989, Comeau et al. in U.S. Pat. No.4,558,276 issued Dec. 10, 1985, all disclose methods of this type.However, these methods suffer from measurement errors due to magneticsaturation of the core material, hysteresis effects in the corematerial, temperature dependent magnetic permeability of the corematerial, and non-linearity of the core material. In addition, themethods are only applicable to the measurement of AC currents. Hastingset al. discloses in U.S. Pat. No. 4,841,235, issued Jun. 20, 1989, theuse of spaced pole pieces with magneto-resistive sensors placed betweenthe pole pieces, and flux shunting pieces between adjacent pole piecesto shunt excessive flux from the sensors and prevent sensor damage. Thepole pieces and shunting pieces also provide magnetic shielding for thesensors from stray magnetic fields. This approach suffers from errorsdue to high permeability materials being present near the sensors,incomplete shielding from stray magnetic fields, and rigid geometricalignment required to maintain calibration.

Karrer et al. disclose in U.S. Pat. No. 6,366,076 issued Apr. 2, 2002,the use of a Rogowski coil together with a magnetic field point sensorsuch as a Hall sensor to create a current sensor with a wide bandwidthcapability. The point sensor is used to measure DC and low frequencycurrents, while the Rogowski coil provides sensitive measurements ofhigh frequency currents. However, this approach does not address errorscaused by stray magnetic fields, and it is generally difficult toseamlessly combine signals covering different frequency ranges.

Several methods have been disclosed to measure current using a number ofpoint sensors arranged around a current carrying conductor. Moriwakidiscloses in U.S. Pat. No. 5,717,326 issued Feb. 10, 1998 the use of twoor four coil or Hall sensors situated around a current carryingconductor to measure the current in the conductor, with half of thesensors oriented with opposing polarity, and the opposing polaritysignals amplified by a difference amplifier to substantially reducestray magnetic field effects. However, the efficacy of the method is notdisclosed, as no mention of Ampere's law is referred to when determiningthe positions of the sensors relative to the conductor, and the deviceis does not clamp on to the conductor. McCormack et al. disclose in U.S.Pat. No. 6,825,650 issued Nov. 30, 2004, the use of more than oneRogowski coils spaced around a circular path the encircles the currentcarrying conductor, with the gap between adjacent coils allowing thepassing through of the current carrying conductor. Also, two concentricrings of coils are disclosed to reduce the effects of stray magneticfields. The two concentric rings do not provide effective cancellationof errors due to stray magnetic fields, and no mention of approximatingAmpere's law is made in the disclosed method. Wakatsuki et al. disclosein U.S. Pat. No. 5,049,809 issued Sep. 17, 1991 the use of a pluralityof magneto-resistive elements connected in series that are disposed on acircular path centered on the conductor and encircling the currentcarrying conductor. The method relies on the use of magneto-resistiveelements, which are nonlinear, saturate and damage easily in highmagnetic fields, and have large temperature sensitivities. Baurand etal. disclose in U.S. Pat. No. 4,709,205 issued Nov. 24, 1987, the use ofa plurality of series-connected air-core coils located on a polygonencircling a current carrying conductor. The method is limited tomeasuring AC currents. Sorenson, Jr. discloses in U.S. Pat. No.6,717,397 issued Apr. 6, 2004, the use of two sets of series connectedcoils positioned on two circular paths of differing radii and centeredon a current carrying conductor. The two sets of coils provide adifference signal that can be used to reduce the errors caused by straymagnetic fields. The method does not eliminate errors due to straymagnetic fields, and is limited to the measurement of AC currents.

Stanley discloses in U.S. Pat. No. 6,531,862 issued Mar. 11, 2003, theuse of multiple current sensors to measure the current in a conductor,by separating the total current into individual sub-conductors, each ofwhich is measured with a current sensor such as a closed loop Hallcurrent sensor. The sensor signals are summed to give the desired totalsignal. Since the noise associated with each current sensor isuncorrelated, the signal to noise ratio of the summed signal improves asthe square root of the number of current sensors used. This approach isunnecessarily complicated.

Stauth, et al. disclose in U.S. Pat. No. 6,781,359 issued Aug. 24, 2004,an assembly consisting of a Hall effect sensor, a magnetic core and anelectrical conductor. The Hall sensor and the core are located near anotch in the conductor. The method suffers from magnetic saturation ofthe core material, hysteresis effects in the core material, temperaturedependent magnetic permeability of the core material, and nonlinearityof the core material all causing measurement errors. In addition, themethod is only applicable to the measurement of AC currents and it doesnot eliminate errors due to stray magnetic fields. Wells discloses inU.S. Pat. No. 5,172,052, issued Dec. 15, 1992, the use of a Hall sensorto measure current in a conductor by locating the sensor near theconductor. Juds et al. disclose in U.S. Pat. No. 6,271,656 issued Aug.7, 2001, the use of a Hall sensor positioned next to a conductor tomeasure current. Lindsey et al. discloses in U.S. Pat. No. 6,555,999,issued Apr. 29, 2003, the use of a point magnetic field sensor placedwithin an insulating column. Bruchmann discloses in U.S. Pat. No.6,472,878 issued Oct. 29, 2002 the use of a U-shaped conductor with theHall sensor placed in close proximity with the conductor to double themagnetic field at the sensor. The methods do not account for, oreliminate, errors caused by stray magnetic fields, conductor size,conductor position or current distribution over the cross-section of theconductor. Alley discloses in U.S. Pat. No. 4,823,075, issued Apr. 18,1989, the use of a Hall sensor placed in a null coil to measure currentin a nearby current carrying conductor. The current in the null coil isadjusted to cancel the magnetic field measured by the Hall sensor,resulting in a null coil current that is proportional to the conductorcurrent. The method does not account for errors caused by stray magneticfields.

Selcuk discloses in U.S. Pat. No. 5,825,175, issued Oct. 20, 1998 theuse of more than one point magnetic field sensor placed in each of twohigh magnetic permeability arms that can be clamped around a currentcarrying conductor. A nulling coil placed around each arm is driven byan adjustable current to null the magnetic field at each sensor element.The adjustable current is a measure of the current in the conductor. Thearms also shield the point sensors from stray magnetic fields. Thismethod suffers from the disadvantages of the errors attributed to a highpermeability material near the sensors, incomplete elimination of errorsfrom stray magnetic fields, and additional errors caused by imperfectmating of the surfaces of the two arms, causing incomplete flux capture.

Temperature compensation of a Hall sensor using a Read only memorylookup table is disclosed by Jerrim in U.S. Pat. No. 4,327,416, issuedApr. 27, 1982. The method uses a lookup table generated by a temperaturecalibration run to provide temperature compensation for the Hall sensor.

Clamp-on and slipover current sensors have been previously disclosed andare well known in the art. For example, Maraio, et al. discloses in U.S.Pat. No. 5,426,360 issued Jun. 20, 1995 the use of a split core of highpermeability material to form a current transformer that can be fastenedaround a conductor without breaking the conductor. This approach suffersfrom saturation of the core at high currents, and errors caused byimperfect contact between the ends of the two halves of the core. Thereluctance of the magnetic circuit is dominated by the air gaps betweenthe halves and repeatable performance is difficult to achieve. Edwardsdiscloses in U.S. Pat. No. 5,057,769, issued Oct. 15, 1991, the use of aC-shaped main coil and a pair of compensating coils at the open ends ofthe main coil to compensate for the opening in the main coil. Thismethod does not compensate for strong stray magnetic fields, andrequires an integration of the output signal to represent the current inthe conductor. In addition, the calibration factor depends on theconductor size and its position in the C-shaped main coil.

Several disclosures address current sensors located far from the currentcarrying conductor. Heroux discloses in U.S. Pat. No. 5,151,649 issuedin Sep. 29, 1992, the use of two sets of triaxial sensor coils tomeasure estimate the current in a conductor far removed from the sensingarray. Strasser discloses in U.S. Pat. No. 4,887,027 issued on Dec. 12,1989, the use of multiple magnetic field sensors to calculate thecurrent in a conductor situated a distance away from the sensors. Thesemethods assume that the conductor generates the dominant magnetic fieldat the sensor array, the geometry is assumed to be well known andunchanging, and ferrous materials are assumed not be in the vicinity ofthe sensors or the conductor. These assumptions lead to large errors inpractical applications.

The power utility industry measures current and voltage to calculatepower flow and energy transferred between suppliers and customers. Thereare several standards defined by the Institute for Electrical andElectronics Engineers (IEEE) and the International Electricity Committee(IEC) that determine the magnitude and phase angle accuracy requirementsfor devices used to measure current when used for revenue metering orsystem protection. For example, ANSI/IEEE Standard C57.13 requires thata current transformer must have an amplitude error of no greater than+/−0.3% and a phase angle error of no greater than +/−15 minutes of arcover a wide range of currents, regardless of temperature, stray magneticfields, conductor size or installation environment. IEC Standard100044-7 has a similar current transformer requirement of +/−0.2%magnitude error and +/−10 minutes of arc in phase angle error. The onlycurrent sensors that can meet these requirements must either replicateor closely mimic Ampere's law. The prior art current sensors mentionedabove that use one or a few point magnetic field sensors cannot meetthese stringent accuracy requirements, and generally have magnitudeaccuracies that fall in the range of 1%-20%. Magnetic field sensors suchas Hall sensors and Magneto-resistive sensors are notoriously inaccuratein conditions as wide-ranging as −50 degrees C. to +85 degrees C., timespans of a decade, large fault currents of >100,000 Amperes, or whensubjected to mechanical stress.

There exists a need for a current sensor that can meet the accuracyrequirements for revenue metering in power utility applications, islightweight, low cost, has a bandwidth from DC to >10 kiloHertz, and canbe clamped in place without having to disconnect the conductor beingmonitored.

SUMMARY OF THE PRESENT INVENTION

Briefly, a current sensor for applications including but not limited toDC, 50 Hz and 60 Hz power lines (or substation bus conductors) isdescribed that consists of a plurality of magnetic field sensorsoriented and located around a current carrying conductor. The magneticfield sensors are preferably Hall effect sensors, although a variety ofother magnetic field sensors can be substituted. The sensors areattached to two printed circuit boards that are placed in two protectivehousings. The two housings are hinged together, allowing the twohousings to close around a continuous conductor without breaking theconductor at either end. The magnetic field sensors are selected to besensitive to one vector component of the magnetic field, and thesensitivity axis of each sensor is oriented to be tangential to a circlecircumscribing, and approximately centered on, the current carryingconductor. As such, the sensors monitor the azimuthal component of themagnetic field, which is directly related to the conductor current. Thenumber of sensors is selected to provide an accurate approximation toAmpere's law. The magnetic field sensor outputs are combined in asumming amplifier. The output of the summing amplifier is passed througha filter circuit to compensate for time delays in the magnetic fieldsensors and the amplifier. The filter output passes through a secondamplifier to provide a desired amplitude gain, resulting in an outputvoltage or current that is substantially proportional to the current inthe current carrying conductor. Additional circuitry is disclosed thatadjusts the output signal from the magnetic field sensors to make theoutput signal substantially immune to changes in temperature.

One advantage of the present invention is that it is very low in weight.

Another advantage of the present invention is that revenue accuracymeasurements can be achieved for power system applications.

Another advantage of the present invention is that relaying accuracymeasurements can be achieved for power system applications.

Another advantage of the present invention is that low cost componentsare used for its fabrication, resulting in a low total sensor cost.

Another advantage of the present invention is that high accuracy isindependent of conductor position within the sensor window.

Another advantage of the present invention is that high accuracy isindependent of conductor tilt relative to the sensor housing.

Another advantage of the present invention is that high accuracy ismaintained over a wide operating temperature range as large as −50degrees C. to +85 degrees C.

Another advantage of the present invention is that high accuracy isindependent of the rotation angle of the housing.

Another advantage of the present invention is that high accuracy isindependent of stray magnetic fields generated by current carryingconductors located nearby.

Another advantage of the present invention is that high accuracy isindependent of the application of mechanical shocks to the sensorhousing.

Another advantage of the present invention is that high accuracy ismaintained because no magnetic core is included in the sensor design.

Another advantage of the present invention is that the sensor canprovide high accuracy measurements of direct currents as well asalternating currents.

Another advantage of the present invention is that the sensor canprovide high accuracy measurements of alternating currents havingfrequencies up to 100 kHz.

Another advantage of the present invention is that high accuracy can bemaintained after extreme temperature excursions as high as 175 degreesC.

Another advantage of the present invention is that high accuracy ismaintained during and after exposure to high currents, since there is nomagnetic core to saturate or damage.

Another advantage of the present invention is that the design lendsitself to simple manufacturing techniques.

Another advantage of the present invention is that the sensor can beclamped onto a conductor, and maintains high accuracy without requiringprecise mating of the clamping members.

Another advantage of the present invention is that multiple sensorarrays can be located in the same housing to provide multiple outputsignals each of which has a different output ratio compared with thecurrent being measured.

Another advantage of the present invention is that no shielding of thesensors from stray magnetic fields is required, since the sensor makes aclose approximation to Ampere's law.

Another advantage of the present invention is that the signal to noiseratio of the sensor output is greater than the signal to noise ratio ofthe each sensor element, since the signals add together linearly withthe number of sensors, but the noise component, being uncorrelatedbetween sensors, adds as the square root of the number of sensors.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a drawing of the current sensor.

FIG. 1B is a block diagram of the current sensor electronic circuit.

FIG. 2 is a plot of the measurement error versus the number of sensorelements used in the design.

FIG. 3 is a schematic diagram of the temperature compensation circuitusing a temperature sensor to adjust the reference voltage.

FIG. 4 is a schematic diagram of the temperature compensation circuitusing the DC offset of the current sensors to adjust the referencevoltage.

FIG. 5A is a schematic diagram of the temperature compensation circuitusing an electromagnet or permanent magnet to generate a signal in oneof the current-sensing magnetic field sensors to adjust the referencevoltage.

FIG. 5B is a drawing of one half of the current sensor, showing thelocation of the electromagnet or permanent magnet.

FIG. 6A is a schematic diagram of the temperature compensation circuitusing an electromagnet to generate a signal in a separate magnetic fieldsensor to adjust the reference voltage.

FIG. 6B is a drawing of one half of the current sensor, showing thelocation of the electromagnet or permanent magnet on the printed circuitboard.

FIG. 6C is a drawing showing the side view of the solenoid coil and themagnetic field sensor used for compensation.

FIG. 7A is a schematic diagram of the temperature compensation circuitusing a separate conductor carrying a known current that is detected bythe array of magnetic field sensors to generate a signal that adjuststhe reference voltage.

FIG. 7B is a drawing showing the side cross section of the currentsensor encircling the conductor, with the secondary conductor positionindicated.

FIG. 8 is a schematic drawing of the use of multiple arrays of magneticfield sensors to provide different sensitivities to the measuredcurrent.

FIG. 9 is a cross-sectional view of the housing showing the trough usedto contain the printed circuit board.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A current sensor for applications including but not limited to DC, 50 Hzand 60 Hz power lines is described that consists of a plurality ofmagnetic field sensors oriented and located around a current carryingconductor. The magnetic field sensors are preferably Hall effectsensors, although a variety of other magnetic field sensors can besubstituted, including but not limited to magnetoresistive, giantmagnetoresistive, or magnetostrictive sensors. The current sensor isshown in FIGS. 1A and 1B. Two printed circuit boards 102 are placed intwo protective, hermetically sealed housings 101 and arranged to form aclosed path around a current carrying conductor 106. The housings arehinged together at hinge 105, allowing the two housings 101 to closearound a continuous conductor without breaking the conductor at eitherend. The two housings are locked together with a fastener at 103. Aplurality of magnetic field sensors 104 are placed on each printedcircuit board. Wiring provides electrical connections between the twoprinted circuit boards. The magnetic field sensors 104 are selected tobe sensitive to one vector component of the magnetic field, and thesensitivity axis of each sensor is oriented to be tangential to a circlecircumscribing, and approximately centered on, the current carryingconductor. The sensors are equally spaced along the circumference of theabove-mentioned circle. As such, the sensors monitor the azimuthalcomponent of the magnetic field, which is directly related to theconductor current through Ampere's law.

The magnetic field sensor outputs 107 are combined in a summingamplifier 108. The output of the summing amplifier is passed through afilter circuit 109 to compensate for time delays in the magnetic fieldsensors and the amplifier. The filter is preferentially a low-passfilter with a cutoff frequency set by the upper frequency range desired,in parallel with a high pass filter having a cut-off frequency wellabove the frequency range of interest for measurements. The low passfilter removes undesired high frequency noise, whereas the high passfilter provides a phase lead compensation for periodic signals tocompensate for a phase lag due to a time delay in the magnetic fieldsensors. The filter output passes through a second amplifier 110 toprovide a desired amplitude gain, resulting in an output voltage orcurrent at 111 that is substantially proportional to the current in thecurrent carrying conductor.

The total number of sensors and the spacing between the sensors alongthe sensing path is determined by the accuracy required and theproximity of other magnetic fields or materials with high magneticpermeability. Computer modeling is used to calculate the expected errorin the magnitude ratio and phase angle of the output signal, when thesensor is located near a second current carrying conductor, near ametallic object having a large magnetic permeability, or when theencircled current carrying conductor is not centered in the sensorhousings, or is not collinear with the central axis of the housings.Limits in the variations in the sensitivity of each magnetic fieldsensor are modeled to determine the variation in sensitivity due tostray magnetic fields and due to rotation of the sensor housings aroundthe current carrying conductor. An example of a calculation is shown inFIG. 2, where the error in amplitude measurement is plotted as afunction of the number of equally spaced sensor elements 104. The errorsare introduced by the presence of a second conductor placed 60 mm awayfrom the current carrying conductor, and carrying a current of 25% inmagnitude of the main current. For this particular disturbance case, thenumber of sensors required to achieve <0.3% errors is at least 6elements. It is to be appreciated by someone skilled in the art thatother perturbation conditions exist, including but not limited toconductor off-centering, conductor tilt, secondary conductor locationsand current levels, variations in responsivity of the sensor elements,conductor diameter, and sensor element position along the sensingcircle.

In the subsequent FIGS. 3-7, the circuit diagrams detail the circuitryon one of the two printed circuit boards comprising the complete currentsensor. It is to be understood that a complete current sensor iscomprised of two of the printed circuit boards, with a summing amplifierthat adds together the outputs of each printed circuit board to providea final output signal for the current sensor. This is shown in FIGS.3-7. Also, the number of magnetic field sensors on each printed circuitboard has been selected for illustration purposes to be six. However,someone skilled in the art will recognize that the number of sensors isadjustable to other values, with the precise number depending on thesize of the individual magnetic field sensors relative to the size ofthe overall current sensor housing, the power supply requirements, andthe desired immunity to external magnetic fields. It is important torealize that four or fewer magnetic field sensors will not be sufficientfor the current sensor to achieve a magnitude accuracy equal to, or lessthan 0.3% and a phase angle accuracy equal to, or less than 0.1 degreesof phase.

The magnetic field sensors are electronic integrated circuits with anoutput signal that is composed of a DC offset voltage that does notdepend on magnetic field intensity, superimposed with a second voltagethat varies with the magnitude and polarity of the magnetic fieldcreated by the electrical current in the conductor (e.g. a 60 Hzsinusoidal signal). To achieve the highest sensitivity, the DC offsetvoltage must be removed from the output signal. The disclosed method isshown in FIG. 3, which shows the circuitry for one of the two printedcircuit boards comprising the current sensor. This is achieved byorienting half of the magnetic field sensors 302 with a positivepolarity (that is, the output voltage increases when a magnetic field isgenerated in the clockwise direction around the current carryingconductor), and half of the magnetic field sensors 311 with the negativepolarity (that is, the output voltage increases when a magnetic field isgenerated in the counter-clockwise direction around the current carryingconductor). The signals from the positive polarity sensors are summedtogether using impedance elements 303, and the signals from the negativepolarity sensors are summed together separately using impedance elements304. Each summed signal has a DC offset voltage that is the average ofthe DC offset voltages of the individual magnetic field sensors, and asignal voltage that is proportional to the average magnetic fielddetected by the magnetic field sensors. Since the same magnetic fieldsensors are used throughout, the DC offset voltages of the two averagedsignals will be effectively equal. The two averaged signals are thendifferenced in amplifier 305 to create an output signal that has no DCoffset voltage, but contains a voltage that is proportional to theaverage magnetic field seen by all of the magnetic field sensors andthus gives a measure of the current flowing through the conductor. Thesignal is then passed through a filter 306 and amplifier 307 to generatean output signal 308. A second identical circuit mounted in a secondhousing provides a second output signal 312 that is substantially inphase with the output 308. The two signals 308 and 312 are summed insumming amplifier 313 to generate an output signal 314 that issubstantially in phase with the measured current and proportional inmagnitude to the measured current. In this way, very small conductorcurrents can be amplified to generate an output signal that is easilydetected. Furthermore, the output signal has a bandwidth that extendsdown to DC currents.

All magnetic field sensors have a sensitivity that varies with theambient temperature, age and mechanical stress. A major challenge forthe use of magnetic field sensors to achieve accurate currentmeasurement is to compensate for these variations to create a currentsensor with a ratio and phase angle accuracy that is substantiallyindependent of temperature, mechanical stress and age. Several methodsto achieve this are described below. In all cases, use is made of thefact that a magnetic field sensor normally provides an output signalthat is proportional to the power supply voltage applied to the sensor.This can be used to compensate the sensor output for sensitivityvariations over temperature, time and mechanical stress.

A first embodiment of temperature compensation is shown in FIG. 3. Theambient temperature of the printed circuit board is detected bytemperature sensor 309 and used to generate a voltage that isproportional to temperature, or a digital number that is proportional totemperature. The error voltage is generated in signal processor 310using an analog amplifier, or it may be generated by a digital look-uptable stored in an electronic memory that is addressed by a numberrepresenting the ambient temperature, and provides a digital number thatis converted to an analog voltage using a conventional digital-to-analogconverter. The error voltage controls a voltage regulator 301 thatgenerates the power supply voltage for all of the magnetic fieldsensors. As the temperature of the printed wiring board varies, thesensitivity of the magnetic field sensors varies. For example, theoutput signal may vary by +3% over a temperature change of 100 degreesC. This is compensated by an equal and opposite variation in the powersupply voltage of −3% over the temperature range of 100 degrees C.,resulting in an output signal that is proportional to the current in thecurrent carrying conductor but substantially unaffected by ambienttemperature. Using this technique, the temperature dependence of theoutput signal can be reduced to 0.2% over a temperature range of 100degrees C.

In another embodiment of temperature compensation shown in FIG. 4, theDC offset voltage of each magnetic field sensor has a temperaturedependence that is similar to the temperature dependence of eachsensor's sensitivity to magnetic fields. The DC offset voltages of thepositive and negative polarity sensors are monitored using impedanceelements 401 to generate a voltage that is the average of the DC offsetvoltages of all of the magnetic field sensors, but substantiallyinsensitive to conductor current or any stray magnetic fields. Thisvoltage is fed to signal processor 402. The error voltage generated bysignal processor 402 may be achieved using an analog amplifier, or itmay be generated by a digital look-up table stored in an electronicmemory that is addressed by a number representing the DC offset voltage,and provides a digital number that is converted to an analog voltageusing a conventional digital-to-analog converter. This voltage controlsa voltage regulator 301 that generates the power supply voltage for allof the magnetic field sensors. As the temperature of the printed wiringboard varies, the sensitivity of the magnetic field sensors varies. Forexample, the output signal may vary by +3% over a temperature change of100 degrees C. The DC offset voltages of the magnetic field sensors alsovary by +0.5% over a temperature range of 100 degrees C. The DC offsetvariation is used to create an equal and opposite variation in the powersupply voltage of −3% over the temperature range of 100 degrees C.,resulting in a DC offset voltage that maintains a constant value as theambient temperature is varied. As a result, the output signal isproportional to the current in the current carrying conductor butsubstantially unaffected by ambient temperature. In this way, the DCoffset voltage variations are used to compensate the sensitivity of themagnetic field sensors as the ambient temperature is varied. Note thatthis method can be used in the presence of DC magnetic fields, becauseboth sensor polarities are used to generate the DC offset voltage. Theresulting DC offset voltage is substantially independent of any appliedmagnetic field.

In a third embodiment of temperature compensation shown in FIGS. 5A and5B, a magnetic field is generated in the vicinity of one or more of themagnetic field sensors. The magnetic field can be a DC field created bya permanent magnet 509 in close proximity to one magnetic field sensor507, or a DC or AC field generated by an electromagnet such as asolenoid 503. The magnetic field sensor 507 should be selected to have atemperature dependence that is substantially the same as the averagetemperature dependence of the entire array of magnetic field sensors. Ifa DC magnetic field is used, then the current sensor can only be used tomeasure AC currents. If an AC magnetic field is used, then the currentsensor can be used to measure DC and AC currents. The magnitude of theextra magnetic field in the region surrounding the magnetic field sensoris kept as stable as possible. For the permanent magnet 509, this isachieved by selecting the permanent magnet material to have thermallystable properties, and includes materials such as Alnico andSamarium-Cobalt. For the solenoid 503, a stable magnetic field isachieved by constructing the solenoid coil mandrel from stable materialsselected from the list including but not limited to Alumina, glass orZirconia, and driving the coil 503 with a constant current generatorformed by sinusoidal oscillator 501 and trans-admittance amplifier 502.The oscillator frequency is preferably selected to lie outside themeasurement bandwidth desired for the current sensor. For example, anoscillator frequency of 1 kHz can be used for a current sensor designedto operate at nominally 60 Hz. The resulting DC or AC signal at theoutput of the individual magnetic field sensor 507 is sent to amplifier504. The signal processor 505 converts the output of amplifier 504 intoan error voltage. If the additional magnetic field is a DC field, thenthe signal processor 504 is an adjustable attenuator or amplifier. Ifthe additional magnetic field is an AC field, then the signal processor504 is comprised of an adjustable attenuator and amplifier fed by asynchronous detector that generates an error voltage. The synchronousdetector performs the function of a narrowband filter, generating anoutput voltage that is proportional to the root-mean-squared amplitudeof the AC signal generated by magnetic field sensor 507 at themodulation frequency of the signal source 501. The error voltage is usedto control a voltage regulator 301 that generates the power supplyvoltage for all of the magnetic field sensors. In this way, the outputsignal of one sensor due to the stable extra magnetic field is used tocompensate the sensitivity of all of the magnetic field sensors as theambient temperature is varied. Note that this will result in an extrasignal being created at the output of the complete current sensor. Forthe solenoid approach, this can be substantially removed by subtractingthe voltage 506 from the current sensor output that is proportional tothe extra magnetic field generated by the solenoid. When using apermanent magnet, the signal 506 is a DC voltage that removes the offsetvoltage generated by magnetic field sensor 507.

In a fourth embodiment of temperature compensation shown in FIGS. 6A, 6Band 6C, a separate magnetic field sensor 604 is placed inside of astable solenoid coil 603 that is in turn driven by a constant currentgenerator. The magnetic field generated by the solenoid coil 603 is anAC field. The solenoid coil 603 and the magnetic field sensor 604 areoriented in such a way that the direction of the generated and detectedmagnetic field is substantially perpendicular to the sensitivity axis ofthe magnetic field sensors 302 already present on the printed circuitboard 606. The magnetic field sensor 604 should be selected to have atemperature dependence that is substantially the same as the averagetemperature dependence of the entire array of magnetic field sensors.The magnitude of the extra magnetic field in the region surrounding themagnetic field sensor 604 is kept as stable as possible. For thesolenoid 603, a stable magnetic field is achieved by constructing thesolenoid coil mandrel from stable materials selected from the listincluding but not limited to Alumina, glass or Zirconia, and driving thecoil 603 with a constant current generator formed by sinusoidaloscillator 601 and trans-admittance amplifier 602. The oscillatorfrequency is preferably selected to lie outside the measurementbandwidth desired for the current sensor. For example, an oscillatorfrequency of 1 kHz can be used for a current sensor designed to operateat nominally 60 Hz.

The resulting AC signal at the output of the individual magnetic fieldsensor 604 is sent to signal processor 605 that converts the output ofthe magnetic field sensor 604 into an error voltage. The signalprocessor 605 is comprised of an adjustable attenuator and amplifier fedby a synchronous detector. The synchronous detector performs thefunction of a narrowband filter, generating an output voltage that isproportional to the amplitude of the AC signal generated by magneticfield sensor 604 at the modulation frequency of the signal source 601.The error voltage is used to control a voltage regulator 301 thatgenerates the power supply voltage for the magnetic field sensors 302and 604. In this way, the output signal of one sensor due to the stableextra magnetic field is used to compensate the sensitivity of all of themagnetic field sensors as the ambient temperature is varied. Note thatthis will not result in an extra signal being created at the output ofthe complete current sensor, which simplifies the technique as comparedwith the approach described in FIG. 5.

In a fifth embodiment of temperature compensation shown in FIGS. 7A and7B, a separate conductor 703 is located in the aperture of the currentsensing device near the measured conductor 707. A precise calibrationcurrent is injected through this conductor by a sinusoidal oscillator701 and trans-admittance amplifier 702 located in the sensor housing708, preferably at a frequency that is well separated from frequenciesoccurring in the main current carrying conductor. The sensor arraydetects the calibration signal as well as the main signal in the mainconductor. A preferred frequency for this signal is >1 kHz, or lowfrequencies such as quasi-DC where the current switches polarity everyfew seconds. The resulting AC signal at the output of the differenceamplifier 305 is sent to signal processor 705 that generates an errorvoltage. The signal processor 705 is comprised of an adjustableattenuator and amplifier fed by a synchronous detector that generates anerror voltage. The synchronous detector performs the function of anarrowband filter, generating an output voltage that is proportional tothe amplitude of the AC current flowing in conductor 703 at themodulation frequency of the signal source 701, and excluding any signalsat other frequencies. The error voltage is used to control a voltageregulator 301 that generates the power supply voltage for all of themagnetic field sensors. In this way, the output signal from the sensorarray due to the stable extra current passing through the sensoraperture is used to compensate the sensitivity of all of the magneticfield sensors as the ambient temperature is varied. Note that this willresult in an extra signal being created at the output of the completecurrent sensor. This can be substantially removed by subtracting avoltage 709 from the current sensor output that is proportional to theextra current flowing in the conductor 703.

More than one set of sensors can be placed along a curve that encirclesa current carrying conductor. As an example shown in FIG. 8, three setsof magnetic field sensors 801, 802 and 803 are placed along curves atthree different radii from the center of the current sensor, formingthree separate sensor arrays on printed circuit boards 804. Since themagnetic field generated by the current carrying conductor variesinversely with the distance from the center of the current carryingconductor, the three sets of magnetic field sensors will produce outputsignals having three different ratios. Different sensor sensitivitiesand different amplifier gains used for each array 801, 802 or 803 canfurther provide adjustability of the ratio of each array's outputsignal. This is a useful feature when a current sensor is required tomeet metering accuracy of 0.3% over a range of 10 Amps to 1000 Amps, aswell as provide accurate representations of the current when faultcurrents occur that can have peak values as high as 100,000 Amps.

As shown in the cross-section in FIG. 9, the current sensor housingconsists of a plate with a trough 903. The printed circuit board 906carrying the magnetic field sensors 905 and other associated circuitryis mounted into the trough and preferably potted in a flexible compound907 selected from the list including but not limited to silicone, epoxy,acrylonitrile butadiene styrene (ABS) and polyurethane. A top lid 901 isfastened to the lower assembly with bolts or other suitable fasteningmeans, interposed between which is a sealing and insulating gasket 902fabricated from a material selected from the list including but notlimited to EPDM rubber, silicone and Viton rubber. The potting andgasket form a hermetic seal to protect the printed circuit board 906from the outside environment.

The housing is preferably fabricated from a metal, but it can befabricated from an insulating material provided that metallic shieldingis placed around the printed circuit boards 906 to provide Faradayshielding of the electronic circuitry from external electric fields. Theuse of a poor electrically conductive material such as bismuth,stainless steel, carbon-filled polymer or metal/carbon filled epoxyprevents substantial eddy currents from being generated, which can causemeasurement errors in both ratio magnitude and phase angle. However, forthese materials the Faraday shielding of the printed wiring board isreduced compared with that provided by highly conductive metals such ascopper or aluminum.

The use of Aluminum as a housing material provides the added benefitthat eddy currents induced in the housing by the magnetic fieldgenerated by the current carrying conductor can be exploited tohomogenize the magnetic field distribution near the magnetic fieldsensors. As shown in FIG. 9, an aluminum top plate is secured to thebottom plate with a means that minimizes the creation of closed currentpaths that encircle the printed circuit board. This can be achieved byusing electrically insulating fasteners and an electrically insulatinggasket material 902 between the top and bottom plates. When measuringcurrents, the magnetic field generated by the current carrying conductoris homogenized by eddy currents induced in the sides, top and bottom ofthe trough containing the printed circuit board, resulting in improvedimmunity to errors induced by external magnetic fields, externalmaterials with high magnetic permeability, and rotation or translationof the current sensing device.

Moreover, eddy currents can be deleterious to device operation when theyencircle the path along which the magnetic field sensors are located. Tominimize this effect, the ends of each plate with trough 900 shown inFIG. 9 are fabricated to reduce the effects of eddy currents on theratio accuracy and phase angle of the current measuring device. The endsof each plate with trough 900 can be modified to have no materialpresent, or they can be modified with a thin slot 904 to prevent eddycurrent paths from encircling the path along which the sensors arelocated. In either case, the open end of each plate with trough 900 isthen filled with an electrically insulating potting compound to form ahermetically sealed surface.

An example of a current sensor is given below. A total of eight Halleffect magnetic field sensors with matched sensitivities to magneticfields are placed on each printed circuit board. Four sensors havepositive orientation, and four sensors have negative orientation. Theoutputs of the sensors are averaged and differenced, and the two printedcircuit board outputs are summed to generate an output voltage. Theoutput voltage is phase shifted with a passive filter circuit. Themagnetic field sensors are temperature compensated using the methodshown in FIG. 3. The resulting current sensor has an aperture opening of2.5 inches, and a sensitivity of 2 volts per kiloamp. The ratio islinear to within 0.1% of reading from 10 Amps to 1500 Amps (AC rms), andhas a noise floor of 1 Amp rms with a bandwidth of DC −5 kHz. The outputphase angle is stable to within +/−5 minutes over all test conditions.The ratio error is +/−0.3% over a temperature range of −40 to +85degrees Celcius. Repeated opening and closing of the clamping mechanismresults in ratio errors of <0.05%. Rotating the current sensor aroundthe current carrying conductor results in errors of <0.1%. Tilting thecurrent sensor relative to the current carrying conductor by +/−30degrees results in ratio errors of <0.3%. The ratio error varies by<0.2% as the conductor is moved anywhere within the sensor's aperture.Varying the size of the conductor from 1 inch to 2 inch diameter resultsin ratio errors of <0.05%. When the current sensor is closed, and placednext to (in contact with) a conductor carrying 1000 Amps, the resultingsignal level is <0.1 Amp of induced signal, resulting in a rejectionratio of >80 dB for currents that do not pass through the current sensoraperture.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. A method to compensate for the temperature dependence and calibrationdrift of a current sensor comprised of a plurality of magnetic fieldsensors positioned around a current carrying conductor, the outputs ofwhich are electronically combined to produce a signal representing thecurrent flowing in said electrical conductor, where the sensitivity tomagnetic fields of each sensor is proportional to the voltage of thepower source that is applied to each sensor, comprising the steps ofselecting a first magnetic field sensor having a temperature dependenceof sensitivity to magnetic fields that is substantially the same as thetemperature dependence of sensitivity for the plurality of magneticfield sensors, positioning first magnetic field sensor inside asolenoidal coil, exciting the solenoidal coil with a known electricalcurrent, electronically processing first magnetic field sensor signal togenerate an error signal that is. proportional to the magnetic fieldcreated by the solenoidal coil while rejecting signals generated byother magnetic fields, and using the error signal to control a voltageregulator that generates the power source for the plurality of magneticfield sensors.
 2. The method in claim 1 where the solenoidal coil andmagnetic field sensor are physically oriented so that the direction ofthe magnetic field and the direction of magnetic field detection aresubstantially perpendicular to the direction of the magnetic fieldgenerated by the current carrying conductor.
 3. The method in claim 1where the magnetic field sensors are selected from the list includingbut not limited to Hall effect, magnetoresistive, giant magnetoresistiveand magnetostrictive sensors.
 4. The method in claim 1 whereelectronically processing first magnetic field sensor signal iscomprised of a synchronous electronic detector.
 5. A device formeasuring electric current comprised of a plurality of magnetic fieldsensors positioned around a current carrying conductor, the outputs ofwhich are electronically combined to produce a signal representing thecurrent flowing in said electrical conductor, where each sensor issensitive to one vector component of the magnetic field generated by theelectric current, where the sensors are positioned along one or morecontinuous closed paths encircling the conductor, where the sensors havesubstantially identical sensitivity along each closed path, where thesensors are equally spaced along the length of each closed path, wherethe vector direction of sensitivity for each sensor is oriented to betangential with the closed path at each sensor location, where thesensitivity to magnetic fields of each sensor is proportional to thevoltage of the power source that is applied to each sensor, where afirst magnetic field sensor has a temperature dependence of sensitivityto magnetic fields that is substantially the same as the temperaturedependence of sensitivity for the plurality of magnetic field sensors,where first magnetic field sensor is positioned inside a solenoidalcoil, where the solenoidal coil is excited with a known electricalcurrent, where first magnetic field sensor signal is electronicallyprocessed to generate an error signal that is proportional to themagnetic field created by the solenoidal coil while rejecting signalsgenerated by other magnetic fields, and where the error signal is usedto control a voltage regulator that generates the power source for theplurality of magnetic field sensors.
 6. The device in claim 5 where thesolenoidal coil and magnetic field sensor are physically oriented sothat the direction of the magnetic field and the direction of magneticfield detection are substantially perpendicular to the direction of themagnetic field generated by the current carrying conductor.
 7. Thedevice in claim 5 where the magnetic field sensors are selected from thelist including but not limited to Hall effect, magnetoresistive, giantmagnetoresistive and magnetostrictive sensors.
 8. The device in claim 5where electronically processing first magnetic field sensor signal iscomprised of a synchronous electronic detector.
 9. A method tocompensate for the temperature dependence and calibration drift of acurrent sensor comprised of a plurality of magnetic field sensorspositioned around a current carrying conductor, the outputs of which areelectronically combined to produce a signal representing the currentflowing in said electrical conductor, where the sensitivity to magneticfields of each sensor is proportional to the voltage of the power sourcethat is applied to each sensor, comprising the steps of selecting afirst magnetic field sensor having a temperature dependence ofsensitivity to magnetic fields that is substantially the same as thetemperature dependence of sensitivity for the plurality of magneticfield sensors, positioning first magnetic field sensor proximate to apermanent magnet, electronically processing first magnetic field sensorsignal to generate an error signal that is proportional to the magneticfield created by the permanent magnet while rejecting signals generatedby other magnetic fields, and using the error signal to control avoltage regulator that generates the power source for the plurality ofmagnetic field sensors.
 10. The method in claim 9 where the permanentmagnet and magnetic field sensor are physically oriented so that thedirection of the magnetic field and the direction of magnetic fielddetection are substantially perpendicular to the direction of themagnetic field generated by the current carrying conductor.
 11. Themethod in claim 9 where the magnetic field sensors are selected from thelist including but not limited to Hall effect, magnetoresistive, giantmagnetoresistive and magnetostrictive sensors.
 12. A device formeasuring electric current comprised of a plurality of magnetic fieldsensors positioned around a current carrying conductor, the outputs ofwhich are electronically combined to produce a signal representing thecurrent flowing in said electrical conductor, where each sensor issensitive to one vector component of the magnetic field generated by theelectric current, where the sensors arc positioned along one or morecontinuous closed paths encircling the conductor, where the sensors havesubstantially identical sensitivity along each closed path, where thesensors are equally spaced along the length of each closed path, wherethe vector direction of sensitivity for each sensor is oriented to betangential with the closed path at each sensor location, where thesensitivity to magnetic fields of each sensor is proportional to thevoltage of the power source that is applied to each sensor, where afirst magnetic field sensor has a temperature dependence of sensitivityto magnetic fields that is substantially the same as the temperaturedependence of sensitivity for the plurality of magnetic field sensors,where first magnetic field sensor is positioned proximate to a permanentmagnet, where first magnetic field sensor signal is electronicallyprocessed to generate an error signal that is proportional to themagnetic field created by the permanent magnet while rejecting signalsgenerated by other magnetic fields, and where the error signal is usedto control a voltage regulator that generates the power source for theplurality of magnetic field sensors.
 13. The device in claim 12 wherethe permanent magnet and magnetic field sensor are physically orientedso that the direction of the magnetic field and the direction ofmagnetic field detection are substantially perpendicular to thedirection of the magnetic field generated by the current carryingconductor.
 14. The device in claim 12 where the magnetic field sensorsare selected from the list including but not limited to Hall effect,magnetoresistive, giant magnetoresistive and magnetostrictive sensors.